Engines may be configured with exhaust gas recirculation (EGR) systems to divert at least some exhaust gas from an engine exhaust manifold to an engine intake manifold. 40 By providing a desired engine dilution, such systems reduce engine knock, throttling losses, in-cylinder heat losses, as well as NOx emissions. As a result, fuel economy is improved, especially at higher levels of engine boost. Engines have also been configured with a sole cylinder (or cylinder group) that is dedicated for providing external EGR to other engine cylinders. Therein, all of the exhaust from the dedicated cylinder group is recirculated to the intake manifold. As such, this allows a substantially fixed amount of EGR to be provided to engine cylinders at most operating conditions. By adjusting the fueling of the dedicated EGR cylinder group (e.g., to run rich), the EGR composition can be varied to include species such as Hydrogen which improve the EGR tolerance of the engine and resulting fuel economy benefits.
Various approaches may be used to reduce the EGR rate in such dedicated EGR systems during conditions when EGR reduction is required. One example approach includes the use of diverter valves for diverting some or all of the exhaust from the dedicated EGR cylinder to an exhaust location. However, the use of diverter valves may be cost prohibitive. In addition, they may have durability issues. Another example approach, shown by Geckler et al. in WO2014005127, shuts off fuel to the dedicated EGR cylinder by deactivating the corresponding fuel injector during engine cold-start conditions, and light engine load conditions. By shutting off fuel to the dedicated EGR cylinder during conditions when less engine dilution is required, the EGR rate can be rapidly lowered.
However, the inventors herein have recognized potential issues with the above approach. As an example, the deactivation of the dedicated EGR cylinder may lead to torque unevenness. For example, even after fuel has been shut off in the dedicated EGR cylinder, due to delays in manifold filling, there may be a corresponding delay in purging EGR from the engine intake. As such, until the EGR has sufficiently purged and due to the lost torque from the dedicated EGR cylinder, the torque output of the remaining engine cylinders may be lower than desired. Then, when the EGR has been used from the system, a higher amount of fresh air may be received in the remaining engine cylinders, causing the remaining engine cylinders to generate more torque than desired. In both cases, the EGR variation leads to a torque excursion. As yet another example, the higher amount of fresh air received during the engine cold-start conditions can lead to a delay in catalyst light-off. Further still, if there is a sudden increase in EGR demand, such as due to a tip-in to higher load conditions, even after the fuel injector of the dedicated EGR cylinder has been reactivated, the same manifold filling delay can cause a delay in ramp-up of EGR to the desired EGR rate. As such, until the EGR has been ramped up to the target rate, there may be torque unevenness.
In one example, the above issues may be at least partly addressed by a method for an engine comprising: during an engine warm-up condition, deactivating a dedicated cylinder group configured to selectively recirculate exhaust to remaining engine cylinders; and adjusting one or more of an intake throttle and a spark timing of the remaining engine cylinders to first increase intake throttle opening while retarding spark timing and then decrease intake throttle opening, while advancing spark timing to maintain overall engine output torque. In this way, torque transients incurred while EGR rate from a dedicated EGR cylinder is ramped-up or down, and while a dedicated EGR cylinder is activated and deactivated can be decreased.
As an example, an engine system may be configured with a single dedicated EGR cylinder for providing external EGR to all engine cylinders. During selected conditions where EGR demand is low, such as during an engine cold-start, during a catalyst warm-up, and/or during light engine load conditions, the dedicated EGR cylinder may be selectively deactivated. For example, fuel to the dedicated EGR cylinder may be shut-off via a deactivatable fuel injector. By deactivating the EGR cylinder, the engine dilution provided by the cylinder is reduced. Deactivating the EGR cylinder also results in engine output torque initially decreasing. Then, as the EGR in the intake manifold is used up and replaced with fresh air, the engine output torque increases. To reduce the torque unevenness involved with the deactivation of the EGR cylinder and ensuing change in EGR, before the deactivating, each of an intake throttle and spark timing may be modulated to build-up a torque reserve before the anticipated negative torque transient. Specifically, before the deactivating, an intake throttle opening may be increased to increase intake airflow to the remaining engine cylinders and build up torque reserve while spark timing of the cylinders is retarded from MBT so as to maintain constant engine torque (despite the increase in airflow). At the time of transition, when the dedicated EGR cylinder is deactivated, spark timing may be advanced to MBT to increase torque and provide a smooth torque transition. Specifically, net engine torque is maintained despite the cylinder deactivation. Further adjustments are then used as the EGR is purged from the engine system. Specifically, as the EGR in the intake manifold falls and is replaced with fresh air, the intake throttle and spark timing may be adjusted to avoid excess torque. Specifically, when the intake EGR level is sufficiently low, the intake throttle opening may be decreased (in relation to the EGR) while spark timing of remaining engine cylinders is retarded to decrease cylinder torque output of the remaining engine cylinders. Thus, cylinder aircharge may be initially increased and then decreased during the cylinder deactivation to reduce torque unevenness.
The dedicated EGR cylinder may be reactivated when reactivation conditions are confirmed, such as when the engine and an exhaust catalyst are sufficiently warmed up and further based on input from a vehicle driver (such as pedal input, or transmission gear change input). Similar throttle and spark timing adjustments may be used when the dedicated EGR cylinder is reactivated (by resuming fuel and valve operation in the dedicated EGR cylinder) to reduce torque transients incurred during the reactivation. For example, intake throttle opening (and thereby cylinder aircharge) may be initially decreased while spark timing is retarded to offset the torque increase from the additional firing cylinder, and then throttle opening may be increased while spark timing is advanced back to MBT.
In this way, external EGR can be varied by activating or deactivating a dedicated EGR cylinder while reducing torque unevenness during the activating or deactivating. By adjusting an intake throttle position and a spark timing of all the remaining engine cylinders before deactivating a dedicated EGR cylinder, a torque reserve may be built to counter the torque drop anticipated at the time of dedicated EGR cylinder deactivation. By subsequently adjusting the intake throttle position and the spark timing based on the change in EGR at the intake manifold, a torque surge anticipated when the EGR is replaced with fresh air can be averted. Specifically, the non-dedicated cylinder torque has to increase to compensate for the deactivation of the dedicated cylinder, but as the EGR is purged from the system, the non-dedicated cylinder torque is maintained, not decreased. The throttle may close a little as the EGR is purged, but the mass of fresh air and non-dedicated cylinder torque output is maintained roughly constant. Thus, by first increasing cylinder aircharge and then decreasing cylinder aircharge during the deactivation of a dedicated EGR cylinder, engine output torque can be maintained throughout the deactivation.
Likewise, during the reactivation, torque in the non-dedicated cylinders is decreased when the dedicated EGR cylinder is activated and then be maintained as the EGR is transported through the induction system. Herein, by first decreasing cylinder aircharge and then increasing cylinder aircharge during the reactivation of a dedicated EGR cylinder, engine output torque can be maintained throughout the reactivation. By reducing torque unevenness during conditions when EGR is ramped in or ramped out from a dedicated EGR cylinder, engine performance is improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.